1. Field of the Invention
The present invention relates to a photomask for use in the fabrication of microelectronic devices, and to a method of manufacturing the same.
2. Description of the Related Art
Photomasks are essential for fabricating microelectronic devices. Photomasks have mask patterns that correspond to the patterns of various components of the microelectronic devices. In a photolithographic process, the photomask is illuminated with light generated by the light source of optical exposure equipment, whereby the image of the mask pattern is transcribed onto a resist formed on an electronic device substrate (e.g., a wafer from which a microelectronic device is fabricated). Photomasks are being miniaturized as such microelectronic devices become more highly integrated. Accordingly, the sizes of the mask patterns are also becoming smaller. However, an optical proximity effect occurs when the scale of the mask pattern is small. The optical proximity effect limits the resolution of the optical exposure equipment. In other words, the optical proximity effect prevents a pattern, having a shape and CD (critical dimension) corresponding to those of the mask pattern, from being formed on the electronic device substrate.
More specifically, as shown in FIG. 1, shot uniformity on the wafer 7 often varies when a mask pattern 5 of a photomask 3 is transferred to a wafer 7 by a light beam 1 emitted from a light source (for example, a KrF excimer laser or an ArF excimer laser) of optical exposure equipment. (Throughout the specification, the term “shot” refers to the entire pattern created on a wafer by scanning a photomask once.) As a result, elements 9 of the pattern have a CD that is different from the CD of elements 8 of the pattern produced according to the design CD. This discrepancy occurs because even though the light beam 1 has a uniform intensity distribution, the light beam undergoes various optical phenomena as it passes through the photomask 3. As a result, the intensity of the light beam (or an exposure dose) varies according to the location at which the light beam impinges the wafer 7. This effect becomes more pronounced as the design rule of microelectronic devices becomes smaller.
A method for correcting CD variations has been used to obtain a pattern whose portions all have the desired CD. This method generally entails creating a wafer CD map or measuring the CDs of a wafer after the wafer is exposed to a light beam to determine the CD variations, and then etching the back side of the photomask (the incident side) based on the CD variations. However, although etching a broad area of the back side of the photomask can correct global variations in the CD to some extent, such etching can not correct for local CD non-uniformity.
On the other hand, a mask that is unable to form a pattern having the desired CD has been conventionally repaired by irradiating the mask pattern with an ion beam. However, such an ion beam repair cannot provide global correction (even though a local correction is possible) and has a limited number of applications.